They have a non-standard source license. I doubt the legal would be excited engineering team to adopt some of these in the codebase.
Although several companies do negotiate custom license agreements with us, it is not very common. Almost all the names here use the standard license: https://nwoods.com/sales/customers.html
Not sure what happened in the grandparent comment, but it’s very much transparent and easy to find.
It also has a neat calculator, so it’s easy to figure out exact pricing for various scenarios, which is quite unlike so many closed libraries I’ve seen.
Running in dev mode it also has a visible watermark on it, which says “GoJs evaluation © Northwoods Software. Not for distribution or production use nwoods.com”, meaning there’s almost zero chance that you wouldn’t notice at dev time that you’d need to license this before putting it into production.